1. Field of the Invention
The field of the invention is fluidized catalytic cracking (FCC) in general and catalyst stripping in particular.
2. Description of Related Art
Catalytic cracking is the backbone of many refineries. It converts heavy feeds into lighter products by catalytically cracking large molecules into smaller molecules. Catalytic cracking operates at low pressures, without hydrogen addition, in contrast to hydrocracking, which operates at high hydrogen partial pressures. Catalytic cracking is inherently safe as it operates with very little oil actually in inventory during the cracking process.
There are two main variants of the catalytic cracking process: moving bed and the far more popular and efficient fluidized bed process.
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts hot, regenerated catalyst. Hot catalyst vaporizes and cracks the feed at 425 C.-600 C., usually 460 C.-560 C. Cracking deposits coke on the catalyst, deactivating it. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and heats the catalyst to, e.g., 500 C.-900 C., usually 600 C.-750 C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and to burn CO, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking is endothermic, it consumes heat. The heat for cracking is supplied at first by the hot regenerated catalyst from the regenerator. Ultimately, it is the feed which supplies the heat needed to crack the feed. Some of the feed deposits as coke on the catalyst, and the burning of this coke generates heat in the regenerator, recycled to the reactor in the form of hot catalyst.
Catalytic cracking has undergone progressive development since the 40s. The trend of development of the FCC process has been to all riser cracking and zeolite catalysts.
Riser cracking gives higher yields of valuable products than dense bed cracking. Most FCC units now use all riser cracking, with hydrocarbon residence times in the riser of less than 10 seconds, and even less than 5 seconds.
Zeolite based catalysts of high activity and selectivity are now used in most FCC units. These catalysts work best when coke on the catalyst after regeneration is less than 0.1 wt %, and preferably less than 0.05 wt %.
To regenerate FCC catalysts to low residual carbon levels, and to burn CO completely to CO2 within the regenerator (to conserve heat and minimize air pollution) many FCC operators add a CO combustion promoter to the catalyst or to the regenerator.
U.S. Pat. Nos. 4,072,600 and 4,093,535, which are incorporated by reference, teach use of combustion-promoting metals such as Pt, Pd, Ir, Rh, Os, Ru and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.
As the process and catalyst improved, refiners attempted to use the process to upgrade poorer quality feeds, in particular, feedstocks that were heavier, and had more metals and sulfur.
These heavier, dirtier feeds pushed the regenerator, and exacerbated four existing problem areas in the regenerator, sulfur, steam, temperature and NOx. These problems will each be reviewed in more detail below.